The invention relates to a covering element for the protection of components in a machine subjected to high thermal load, in particular of components in a gas turbine. The invention relates, furthermore, to an arrangement with a covering element and with a carrying structure.
Components in a machine subjected to high thermal load are exposed to high temperatures during the regular operation of this machine. In a thermal machine, in particular in a gas turbine, a hot medium, for example a hot gas, subjects to a very high thermal load, primarily the surfaces, limiting the hot medium and the associated components. Furthermore, as a result of the transport of heat through these limiting surfaces, such as occurs, for example, in the form of heat conduction or heat radiation, even components which are not directly exposed to the hot medium and are often installed in the casing of the machine are subjected to high thermal loads. The components exposed to the hot medium thus perform two functions: enclosing the hot medium and protecting other, possibly less heat-resistant components from overheating or thermal destruction. Consequently, above all, material properties and the design and mounting of these components subjected to high thermal load must satisfy stringent requirements. Moreover, requirements regarding the coolability of such components must often also be taken into account.
For example, when a gas turbine is in operation, loads arise from mechanical stress (for example, due to internal pressure, centrifugal force, external forces and moments) and as a result of thermal stresses which occur because the thermal expansion of components in the event of temperature differences is prevented. Whereas, during steady-state operation, the temperature differences and therefore the thermal stresses are generally low, as compared with the mechanical stress, during transient operation in the event of load changes and in start-up and shut-down actions the transient thermal stresses are usually decisive, since load changes are necessarily associated with temperature changes. In the case of high working temperatures and large temperature differences between the individual load states, therefore, corresponding thermal expansions occur which affect primarily the casings and the rotors.
U.S. Pat. No. 3,892,497 describes an axial gas turbine with an inner and an outer casing insert. Guide blades and moving blades are arranged along a turbine axis in the gas turbine. A guide blade has in each case a platform (guide blade root) which serves for fastening the guide blade to the inner casing insert. Between in each case two adjacent guide blades spaced axially from one another, a guide ring is arranged on the inner casing insert in such a way that the guide ring is contiguous to the corresponding platforms of the guide blades. The platforms and guide rings are held from inside by the inner casing insert and are connected to the latter by a carrying element. Each carrying element is in this case connected fixedly to the inner casing insert by means of a combination consisting of a locking plate with a screw engaging into the inner casing insert.
The platforms of the guide blades and the guide rings have grooves into which the carrying element engages. A carrying element in this case engages into a groove either in a platform or in a guide ring, engagement taking place in the axial direction in each case at the edge of the platform or guide ring. This fastening to some extent allows relative thermal expansion and contraction between mutually contiguous components in the axial direction and, furthermore, permits simplified assembly and maintenance of the gas turbine. Moreover, a fastening for a guide ring may be gathered from the patent specification, in which a rigid connection to the guide ring is made directly by means of a fixing screw guided radially through the inner casing insert. In this case, the fixing screw secures the guide ring locally at a point between the axial edges of the latter. This embodiments results, when the guide ring is under thermal load, in considerable local thermal stresses in the axial direction and, above all, in the radial direction, since thermal expansions are possible only to a very restricted extent.
An object on which the invention is based is to specify a component capable of being subjected to high thermal load and at the same time of being cooled as efficiently as possible. The component, in this context, is to be suitable for use in the case of high working temperatures and large temperature differences between various states of load. Another object of the invention is to specify an arrangement with a component and with a carrying structure, which makes it possible, in particular, to fasten the component in the carrying structure in a way which is tolerant to thermal expansion.
The first-mentioned object is achieved, according to the invention, by means of a covering element which has a longitudinal axis and a transverse axis, comprising a wall with a hot side capable of being exposed to a hot medium and with a cool side which is located opposite the hot side and which has a cooling surface capable of being acted upon by a coolant, and further comprising a first bearing region, contiguous to the wall along the longitudinal axis and having a first bearing surface, and a second bearing region, located opposite the first bearing region along the longitudinal axis and having a second bearing surface, and further comprising a first edge region contiguous to the wall along the transverse axis and a second edge region located opposite the first edge region along the transverse axis, there being provided on the cool side a holding element which is arranged between the first and the second bearing region.
The invention proceeds from the notion that a component in a thermal machine, said component being exposed to a hot medium, for example a hot gas or steam, is subjected to very high thermal load by the temperature of the medium. These high temperatures or large temperature changes are associated with heat-induced deformations, above all thermal expansions, which are to be taken into account in the design and mounting of such components. The invention affords a novel possibility for designing and arranging components in a way which is tolerant to thermal expansion in machines subjected to high thermal load.
An above covering element forms, with its hot side capable of being exposed to the hot medium, a defined limitation of the hot medium, for example of the hot gas, in the combustion chamber or in the flow duct of a gas turbine. Furthermore, the covering element, as a component capable of being subjected to high thermal load, serves for the protection of further, possibly less heat-resistant components which are not exposed to the hot medium directly and are arranged in the casing of the thermal machine, in particular of the gas turbine. In this function, the covering element prevents the thermal overloading or even destruction of these components. Provided on the cool side of the covering element is a holding element which is arranged between the first and the second bearing region. The holding element is a fixed integral part of the covering element and has the task of ensuring an additional hold between the first and the second bearing region. The covering element is in this case held via the holding element from the cool side in such a way that, in particular, forces directed perpendicularly to the wall, for example as a result of mechanical and/or thermal load on the wall, can be absor-bed efficiently and, if appropriate, also transmitted efficiently.
At the same time, very good cooling properties of the cooling element can be ensured. This is implemented in that the first and the second bearing region are contiguous to the wall along the longitudinal axis. The side of the wall which is located opposite the hot medium is thereby available virtually completely as a cooling surface. By virtue of this design, the cooling surface is capable of being acted upon uniformly by a coolant, for example cooling air, with the result that highly homogeneous cooling becomes possible.
It also has a particularly advantageous effect on the use of coolant, since the cooling surface is designed as a coherent surface and, as a result, the coolant, insofar as it is supplied at a point on the cool side, can reach all the regions of the cooling surface. Additional coolant feeds or coolant leadthroughs therefore become unnecessary, which is highly advantageous, above all, in light of the production costs.
The good cooling properties of the covering element also have a particularly beneficial effect on temperature distribution within the wall of the covering element. Consequently, temperature gradients occur essentially only perpendicularly to the cooling surface, that is to say from the hot side in the direction of the cool side. Thermal stresses along the longitudinal or transverse axis of the covering element, which could possibly induce cracks, are thereby as far as possible avoided.
The proposed covering elements proves highly advantageous also in terms of mechanical stability. This is primarily in regard to the forces which occur due to possible pressure differences which may prevail between the hot side and the cool side of the covering element. Both the mechanical load and the above-described thermal load on the covering element lead to a deformation of the wall which is normally manifested in a flexion of the wall in the direction of the hot side. This effect is restricted to a defined amount by virtue of the invention.
Preferably, a further holding element is arranged on the cooling surface, on the first or on the second edge region. A further holding element affords the possibility of giving the covering element an additional hold at a further point from the cool side of the wall. The overall load due to mechanically and/or thermally induced forces perpendicular to the wall is thereby distributed to a plurality of holding elements, with the result that the load per holding element becomes correspondingly lower. Possible flexions of the wall in the direction of the hot side as a result of these forces are thereby either further restricted or can be limited to a predetermined amount by virtue of an appropriate arrangement of the holding element. Furthermore, the good cooling properties of the covering element are maintained due to the further holding elements, that is to say, above all, the design of a coherent cooling surface on the cool side. Various combinations of two holding elements can be implemented, which lead to the same desired result in terms of a predetermined maximum deformation of the wall. This affords a certain amount of freedom with regard to the arrangement of the holding elements.
The holding element preferably has a holding bearing surface. The holding element has, also preferably, a recess, in particular a groove, for engagement into a carrying structure. By virtue of this design, it is possible, via the holding element, in combination with the first and the second bearing region and also with a carrying structure, to implement an arrangement tolerant to thermal expansion, with the covering element and with a carrying structure. The production of the holding bearing surface as a subsurface of the recess, in particular of the groove, in the holding element can be carried out in a simple way in manufacturing terms. The recess could be produced, for example, by the milling of a groove or, where a casting is concerned, by laying bare by means of a simple core during casting. The holding bearing surface serves for absorbing the forces as a result of thermal and/or mechanical load on the covering element and for transmitting them effectively to a carrying structure. The occasionally considerable forces are not point-transmitted by the holding bearing surface, but are distributed over an area. Thus, for a given termal or mechanical load, the load per area can be limited, by appropriate dimensioning of the holding bearing surface, to an amount adapted to the material properties of the covering element.
The wall preferably has a wall thickness of between about 1.0 mm and 5.0 mm, in particular between about 1.5 mm and 3.0 mm. The wall is consequently made comparatively thin, as compared with the first and the second bearing region of the first or the second edge region of the covering element. Depending on the application, during operation, the temperature difference between the wall""s hot side acted upon by the hot medium and the wall""s cool side acted upon by the coolant may be very large. For example, when the covering element is used in a gas turbine, temperature differences between the hot gas and the coolant, in particular the cooling air extracted from the compressor of the gas turbine, of up to 800xc2x0 C. may occur. It is therefore of decisive advantage to make the wall as thin as possible, so that the temperature gradient between the hot side and the cool side of the wall becomes as high as possible and the heat can be discharged very efficiently, with the smallest possible amount of coolant being used. An efficient heat discharge takes place predominantly by means of the coolant. A small fraction of the heat flow flowing from the hot side into the wall may also be diverted along the longitudinal axis and the transverse axis into the first/second bearing region and the first/second edge region of the covering element, since these regions constitute an additional heatsink because their cross section is greater than that of the wall.
The cooling surface preferably has a supporting structure for increasing the rigidity and thermal conductivity. The increase in the rigidity of the covering element by means of the supporting structure on the cooling surface has a highly advantageous effect on the prevention of deformations, in particular of deformations and flexions of the wall in the direction of the hot side of the wall. Furthermore, this supporting structure has the effect of enlarging the effective cooling surface, thus leading to an increase in cooling efficiency. In addition to enlarging the effective cooling surface, the supporting structure ensures an improved intermixing of coolant at different temperatures in the immediate vicinity of the cooling surface. As a result, on average, the temperature on the cooling surface decreases, and the temperature gradient and, correspondingly, the transport of heat by the coolant are increased. In addition, since the cross section of the supporting structure is larger than that of the wall, thermal conductivity along the supporting structure is increased somewhat.
Preferably, the supporting structure is formed by at least one longitudinal rib along the longitudinal axis on the cooling surface. Also preferably, the supporting structure has a further longitudinal rib which is formed along the longitudinal axis on the cooling surface. The design of the supporting structure in the form of one or more longitudinal ribs is a solution which is highly beneficial in terms of production and which, for example in the case of a casting, can be implemented simply and cost-effectively. As regards the improved thermal conduction properties, this design leads to a transport of heat through the longitudinal ribs in the direction of the first and the second bearing region of the covering element. At the same time, the longitudinal ribs increase the rigidity of the component, which, in turn, is advantageous in terms of possible deformations, in particular flexions of the wall from the cool side toward the hot side, under thermal or mechanical load.
Preferably, at least two longitudinal ribs spaced in the direction of the transverse axis are connected to a holding element. By virtue of this design, the holding element may be interpreted, as it were, as part of the supporting structure. This version serves for increasing the rigidity and for increasing the thermal conductivity, but, above all, the mechanical and thermal stability of the covering element under high thermal and/or compressive load. It is advantageous, once again, that this version can be implemented in a simple way in manufacturing terms.
Preferably, the number and arrangement of the holding elements are defined by a predetermined thermal flexion of the wall. Also preferably, the predetermined thermal flexion is 0.1 mm to 1.0 mm, in particular 0.3 mm to 0.7 mm. The thermal flexion which occurs depends, in this case, on the thermal load and/or compressive load on the covering element and on its material properties and also on the design, predominantly in terms of the number and arrangement of the holding elements. In the case of a typical temperature difference between the hot side and the cool side of the covering element of approximately 800xc2x0 C., such as occurs, for example, in a steady-state gas turbine, the limits specified above for the thermal flexion are reasonable values. In an actual application, it will be necessary to find a suitable configuration by means of computer-assisted optimization of the concurrent requirements between the flexion of the wall, on the one hand, and, in accompaniment with this, a number and arrangement of holding elements on the cooling surface, and an acceptable restriction of the effective cooling surface by the holding elements, on the other hand. A proposed concept therefore affords very high flexibility with regard to adaption to an actual set object.
Preferably, at least two holding elements are arranged, spaced from one another, along the transverse axis. Also preferably, at least two holding elements are arranged, spaced from one another, along the longitudinal axis. In the case of covering elements which are dimensioned such that they extend predominantly along the longitudinal axis or along the transverse axis, a plurality of holding elements are provided along the respective preferential axis. This version is closely adapted to the symmetry properties of the covering element and, in the case of a predetermined thermal flexion of the wall, manages with as small a number of holding elements as possible. As regards covering elements which extend appreciably along both a longitudinal axis and a transverse axis, holding elements are arranged preferably in both dimensions, in order to achieve the desired effect. It is advantageous, in this case, if the holding elements are arranged, spaced from one another, and the cooling surface thus always remains a coherent surface in all the embodiments. The cooling air can thereby flow, unimpeded, from one point on the cooling surface to another point on the cooling surface, and there is no need for additional coolant feeds or coolant leadthroughs.
The object based on an arrangement is achieved, according to the invention, by means of an arrangement with a covering element according to one of the above versions and with a carrying structure which has a longitudinal axis, a transverse axis and a first receiving region arranged along the longitudinal axis and having a first receiving surface and also a second receiving region located opposite along the longitudinal axis and having a second receiving surface, and a carrying element with a carrying surface, the first receiving region being contiguous to the first bearing region and the second receiving region to the second bearing region, and the holding element and the carrying element overlapping one another, the holding bearing surface and the carrying surface being located opposite one another.
Preferably, without any thermal load, in particular at room temperature, the holding bearing surface and the carrying surface are spaced from one another by a gap. The covering element is usually inserted into the carrying structure at room temperature. Since the first receiving region is contiguous to the first bearing region and the second receiving region to the second bearing region, the covering element is already held in a carrying structure. The spacing of the holding bearing surface of the holding element and the carrying surface of the carrying element by means of a gap proves to be highly beneficial in terms of the mounting of the covering element in the carrying structure. When the thermal machine, in particular a gas turbine, is in operation, that is to say under high thermal and mechanical load, the wall of the covering element tends to flex in the direction of the hot side. The holding bearing surface on the carrying surface therefore come into congruence, and the forces resulting from the thermal load are absorbed effectively. The spacing, selected at room temperature, between the holding bearing surface and the carrying surface is decisive as to the thermal load at which the holding bearing surface and the carrying surface come into congruence and therefore as to the thermal flexion of the wall which occurs. In the thermally highly loaded state, the covering element is thereby held firmly in the carrying structure, the thermal flexion of the wall in the direction of the hot side being predeterminable, in particular being capable of being restricted to a maximum value.
Preferably, one configuration formed between the receiving region and the bearing region contiguous to it is designed as a fixed bearing and the other configuration as a loose bearing. This design proves to be particularly advantageous, since the arrangement with a covering element and with a carrying structure constitutes, in general, a system with high-grade mechanical redundancy. This system has a series of bearing configurations which are formed by the receiving regions and the contiguous bearing regions and, furthermore, by the mutually overlapping holding elements and carrying elements. The design with a fixed bearing and a loose bearing ensures that the covering element is mounted in a simple way in the carrying structure in the thermally nonloaded state.
Moreover, thermal expansion of the covering element along the longitudinal axis becomes possible. Thermal expansion takes place, in the case of a temperature rise, from the fixed bearing in the direction of the loose bearing. The fixed bearing configuration is in this case designed in such a way that, even in the case of only a slight temperature rise, as compared with room temperature, the corresponding receiving region and the bearing region contiguous to it come into contact with one another. By contrast, the loose bearing is dimensioned such that, even at very high temperatures, such as may occur when a gas turbine is in operation, the covering element can still expand along the longitudinal axis. This results here, in particular, in the advantages of a simple mounting and the arrangement, tolerant to thermal expansion, of the covering element in a carrying structure. Thermally induced deformations, in particular thermal expansions, are taken into account and, at the same time, the covering element is held firmly in the carrying structure via the holding elements at high temperatures.
Preferably, the fixed bearing has a tolerance of between about 0.2 mm and 0.5 mm. Also preferably, the loose bearing has a tolerance of between about 4.0 mm and 10.0 mm.
Preferably, the covering element and the carrying structure are arranged in a thermal machine, in particular in a gas turbine. The fastening concept tolerant to thermal expansion is particularly appropriate with regard to a platform for fixing a gas turbine blade, to a guide ring in a gas turbine, to a head platform of a guide blade of a gas turbine or to a heat shield element in the combustion chamber of a gas turbine. Where a gas turbine is concerned, a distinction is made between guide blades and moving blades which are in each case arranged on rings radially to the axis of rotation of the gas turbine. A guide blade has a platform which is arranged for fixing the guide blade on the inner turbine casing, in particular on the guide blade cascade segment. A moving blade is fastened via a platform on the turbine rotor arranged along the axis of rotation. A guide ring is arranged as a wall element in a gas turbine between the platforms of two successive guide blades spaced axially from one another. The outer surface of the guide ring is exposed to the hot medium, in particular the hot gas, and is spaced in the radial direction from the outer ends of the rotating moving blades by a gap. In addition to the applications in a gas turbine, further embodiments of the covering element are possible, for example as a wall element in furnaces, in combustion chambers or in vessels capable of being filled up with hot media.